Consumption of dietary soy has been associated with reduction in risk of several chronic diseases, such as: cardiovascular disease, osteopenia in post-menopausal women, and several types of cancers. Soy contains many phytochemicals including phytates, saponins, protease inhibitors, fiber, phytosterols, phenolic compounds, and isoflavones. Many of these phytochemicals may be involved in the health beneficial effects associated with soy consumption. Soy contains high levels of the isoflavone genistein and considerable research has been conducted to relate the beneficial health effects from consumption of soy with genistein. However, genistein is a known phytoestrogen and the possibility that consumption of diets high in genistein may be detrimental to women at risk for estrogen-dependent breast cancer requires evaluation. We have demonstrated that dietary consumption of genistein results in: growth of implanted estrogen-dependent human breast cancer (MCF-7) cell tumors in ovariectomized athymic mice, enhancement of mammary development and uterotrophic effects in ovariectomized rodents and stimulation of growth of cultured MCF-7 cells at low concentrations of genistein. We hypothesize that various forms of dietary genistein will produce blood and tumor concentrations of genistein sufficient to stimulate growth of estrogen-dependent human breast cancer (MCF-7) cells in vivo. To test this hypothesis, we are proposing to conduct a series of feeding studies to determine the potential of various levels of dietary genistein to stimulate growth of MCF-7) cell tumors implanted into ovariectomized athymic female mice. We will also evaluate the interaction of genistein with estradiol for its potential to alter growth rate of MCF-7 cell tumors in athymic mice. An evaluation of dietary genistin, the glycoside in legumes, for its potential to enhance tumor growth in ovariectomized athymic mice will be conducted. We will also compare diets containing genistein with diets containing soy protein isolates with equal contents of genistein to evaluate the interaction of genistein with other compounds in soy. Results of the proposed studies will elucidate the role of genistein from soy on growth of estrogen-dependent tumors in vivo.